1. Field of the Invention
The present invention concerns the highly accurate control (to within a few nanometers) of movement of an optical component of a cat's eye delay line in a stellar interferometer system. It is more generally concerned with an interferometer device such as a FOURIER TRANSFORM SPECTROMETER in which, in addition to dynamic stabilization of the optical component during programmed movement to eliminate the effect of vibration, disruptive optical path differences generated independently of the optical component and its movement in the remainder of the interferometer device or exterior thereto are compensated.
Programmed vibration-free displacement of this kind is needed, for example, to control an optical delay line within a coherent recombination type stellar interferometer. In this case, where the distance (called the base) between the telescopes is in the order of 100 meters, equalizing the optical paths, despite the rotation of the Earth, requires displacement of the retro-reflector device of the optical delay line over a distance of several meters, at a speed of several tens of millimeters per second, with an average positional accuracy of a few microns and with an instantaneous accuracy (during a setting time of a few hundredths of a second) of a few nanometers (amplitude of the vibrations): the constraints are therefore more severe in respect of the dynamic error than in respect of the static error.
Such displacement characteristics are also necessary if the optical paths are equalized by moving the telescopes themselves.
2. Description of the Prior Art
The 1986 MIT thesis by R. W. HOLM entitled "A High Speed High-Resolution Optical Delay Line for Stellar Interferometry Applications" discloses a device for displacing a "cat's eye" in a delay line comprising a slave carriage moving on rails and comprising a chassis mobile on flexible pivots to which the cat's eye is fixed.
The secondary mirror of the cat's eye is carried by a piezo-electric actuator. The position of the mobile chassis relative to the carriage is controlled by a displacement detector and a loudspeaker voice coil type linear actuator. Finally, the carriage is moved by a stepper motor.
The optical path introduced by the delay line is measured by a laser interferometer. The error signal is injected into the control loop of the piezo-electric actuator. Saturation of this actuator is avoided by the action of the linear actuator and saturation of the linear actuator is avoided by the action of the stepper motor (hence the above use of the term "slave"). There are therefore three control loops nested one within the other.
This displacement device makes it possible to achieve vibration-free displacement of the cat's eye similar to friction-free displacement. It offers good performance because the error signal corresponds directly to the magnitude to be controlled, that is to say to the optical path.
On the other hand, it is ill suited to high speeds because the laser measurement system would then have to have a resolution of a few nanometers while the ratio of the maximum speed of variation of the optical path of the carriage to the resolution is limited by the electronic circuitry for reasons of feasibility.
Also, the piezo-electric actuator cannot apply sufficient force for this device to be applicable to the movement of large masses such as telescopes.
Finally, this device is costly because of the presence of the three control loops and because of the need to have a high-performance laser measurement system which can only function correctly in a vacuum.
A device of this type is described in the publication: P. CONNES 1975 Applied Optics, Volume 14 No 9, p 2067-2084.
The Kachler et al U.S. Pat. No. 5,008,606 describes a device for programmed movement of a carriage carrying via a flexible coupling a payload which must be moved without vibration over a long travel and at high speeds. This device is of the kind used for the delay lines of a stellar interferometer and is free of the drawbacks mentioned above.
This document discloses a vibration-free displacement control device suitable for static and dynamic control of a mobile optical element movable along one direction in an interferometer device comprising a guide track, a carriage carrying a payload including said optical element and movable along said guide track, a motor by which said carriage is driven along said guide track, drive control means connected to said drive motor and adapted to move said carriage according to a predetermined law, flexible linkage means between said payload and said carriage enabling relative movement between said payload and said carriage parallel to said direction of displacement of said optical element, and at least one actuator adapted to act on at least part of the optical element under the control of at least one stabilization control loop connected to at least a second sensor, the device being characterized in that the stabilization control loop is independent of the drive control means, in that the second sensor is an accelerometer carried by the payload and responsive to acceleration of the payload parallel to said direction of displacement of said optical element, in that the actuator acts on all of the payload while bearing on the carriage, in that the stabilization control loop includes filter means for filtering signals from the accelerometer, and in that the drive control means comprises a control loop connected to a position sensor adapted to sense the position of the carriage on the guide track.
In practice the payload is provided with an accelerometer measuring acceleration in the direction of displacement of the carriage and carries a primary element of a linear electromagnetic actuator the secondary element of which is fixed to the carriage and the force applied by the linear motor is controlled by the signal from the accelerometer so that vibrations of the carriage are not transmitted to the payload.
Preferred features of the document included:
the filter means of the stabilization control loop was a second order filter having a transfer function F(p) of the form: ##EQU1## the accelerometer was of the piezo-electric or slaved pendulum type, the actuator was of the voice coil type, PA1 the guide track used crossed rollers, PA1 the drive motor was a brushless linear motor, PA1 the position sensor was an incremental type sensor, PA1 the drive control loop was a digital position/speed control loop, PA1 the optical element was a "cat's eye" retro-reflector in a delay line, PA1 the optical element was one of the telescopes of a stellar interferometer, mobile towards a recombination station. PA1 slow drift in pointing in the case of an interferometer device on board a spacecraft, PA1 vibrational deformations (at frequencies in the order of 10 Hz, for example) possibly due to flexibility of the spacecraft, PA1 atmospheric turbulence in the case of ground observations. PA1 a carriage carrying a chassis to which said primary mirror is fastened and which is movable along said guide track, a motor for driving said carriage along said guide track, a first drive motor control loop connected to a position sensor adapted to sense the position of said carriage on said guide track to move said carriage in accordance with a predetermined set point law, PA1 flexible linkage members coupling said chassis to said carriage enabling relative movement between said carriage and said chassis parallel to said direction, at least one actuator adapted to bear onto said carriage and act on said chassis under the control of a second control loop independent of said first control loop and connected to a second sensor in the form of an accelerometer carried by said chassis and responsive to acceleration of said chassis parallel to said direction, PA1 wherein said secondary mirror is coupled to said chassis by a piezo-electric actuator acting on said secondary mirror parallel to said direction and controlled by a third control loop receiving as input an optical path error signal generated in said recombination station and delivering also to at least one of said first and second control loops an actuator desaturation signal. PA1 space qualification was problematical because of the laser (service life, reliability, sensitivity to environmental constraints), PA1 large travels (&gt;30 meters) were difficult to obtain because of diffusion of the laser light over the reflective surfaces in the cat's eye, PA1 the cost of the laser measuring system and of adjusting the three nested loops, PA1 the risk of mixing of the laser signal with the stellar signals and therefore of pollution of the stellar astronomic signal.
This document also proposed a stellar interferometer comprising one or more movement control systems of the aforementioned type.
In practice the novelty of the invention resided notably in the fact that, to damp very effectively vibrations of an optical element mobile along a given path with given kinetic properties, it taught, in a way that went counter to the received wisdom of those skilled in the art, dispensing with any interconnection between the drive and stabilization control systems; surprisingly, the resulting great simplicity (and high reliability) were combined with excellent damping.
This solution enabled very accurate programmed movements of the cat's eye over long distances (up to 100 m) without requiring laser measuring techniques.
An object of the present invention is to improve the teachings of U.S. Pat. No. 5,008,606 by further enabling real time compensation of disturbances due to random and unknown optical path differences generated upstream or downstream of the delay line, possible sources of such disturbances including: